1. Field of the Invention
The present invention relates to a backlighting device and a method of manufacturing the same. More specifically, the present invention relates to an edgelight type backlighting device suitably used for a liquid crystal display apparatus and the like, and also relates to a liquid crystal display apparatus including the backlighting device.
2. Description of the Related Art
A class of backlighting devices for liquid crystal display apparatuses is generally known in which light generated from a capillary-like light source element (e.g., a cold cathode tube) disposed along a side face of a light guide unit enters into the light guide unit so as to be emitted through one face of the light guide unit. In such a backlighting device, the light source element and the light guide unit do not vertically overlay each other, enabling reduction of the size of the backlighting device along the thickness direction. In such a backlighting device, however, a space for disposing the light source element is required outside the light guide unit, which may become a dead space, i.e., a space which cannot be utilized as an effective viewing area, in a liquid crystal display apparatus incorporating such a backlighting device. Currently, there is an increasing need for an enlarged display area and a reduced module size, especially in on-vehicle liquid crystal display apparatuses and liquid crystal display apparatuses for mobile terminals.
Japanese Laid-open Publication No. 9-5742 discloses a relatively compact backlighting device having a structure shown in FIG. 12. The liquid crystal display apparatus 100 shown in FIG. 12 includes a liquid crystal panel 110, and a backlighting device 120 disposed under the liquid crystal panel 110. The backlighting device 120 includes a housing 122 for a light source element L, in the form of a notch provided in portion of each lateral end 121a of a light guide unit 121. The light source element L is disposed within the housing 122. A casing 123 covers the side faces and the bottom face of the light guide unit 121. A light diffusion panel 124 and a prism sheet 125 are provided between the upper face of the light guide unit 121 and the liquid crystal panel 110.
In the conventional backlighting device 120, the light guide unit 121 is composed of a transparent resin and includes a narrow portion 126 at each lateral edge 121a where the light guide unit 121 meets the upper face of the housing 122 for light source element. The narrow portion 126 is provided by mixing a particulate material that has a different refractive index into the transparent resin. Between the narrow portion 126 and the housing 122 for light source element, a light amount adjusting filter means 127 is formed. The disclosure of Japanese Laid-open Publication No. 9-5742 teaches that both the luminance unevenness and the color irregularity of the backlighting device 120 were eliminated by providing the light amount adjusting filter means 127 on the narrow portion 126 of the light guide unit 121.
As the light amount adjusting filter means 127, the following structures (1)-(4) are disclosed.
(1) a reflective PET film (thickness 188 microns), having apertures with diameters of 0.2 mm or less directly formed therein, where the aperture ratio is 12%.
(2) a reflective PET film (thickness 75 microns), with white ink dots being printed exclusively in a portion immediately above the light source element.
(3) a transmissive PET film with a thickness of 100 microns, bearing a dotted layer of aluminum deposited on one side thereof, where the aperture ratio is about 12%.
(4) a transmissive PET film with a thickness of 100 microns, bearing a dotted layer of aluminum deposited on both sides thereof, where the aperture ratio is about 10%.
If dot printing is used in the above-mentioned method disclosed in Japanese Laid-open Publication No. 9-5742, however, for example, problems may occur during the production of the backlighting device, such as unwanted variation in luminance and color caused by uneven printing and irregular ink distribution due to the abrasion of the silk screen (currently, a silk screen must be replaced after having been used for ten thousand shots). On the other hand, if aluminum layer deposition is used, there are problems such as changes in the dot shape caused by degradation of the deposition mask, chromaticity shifting due to the oxidation of the deposited aluminum layer, and peeling of layers due to insufficient adhesion to the PET sheet.
Furthermore, for producing the light amount adjusting filter means 127, some cost is incurred for, e.g., a dot printing machine, a vacuum deposition machine, and deposition jigs. Also, substantial cost is incurred that is associated with the abrasion of the machines and product inspection. Furthermore, the provision of the light amount adjusting filter means 127 on the narrow portion 126 of the light guide unit 121 may present serious cost problems associated with the cost for assembling, the cost for defects during the assembling due to defective parts or improper assembling, and the inspection cost.
Japanese Laid-open Publication No. 8-166513 discloses a light guide unit 130 as shown in FIG. 13A. A transmission adjusting section 131 is provided on the inner surface 130b of an extension 130a of the light guide unit 130 for adjusting the light transmittance. The transmission adjusting section 131 also has a light diffusion function. Japanese Laid-open Publication No. 8-166513 explains that linear advancement of the light from the light source element L and into the transmission adjusting section 131 is ensured because the inner surface 130b of the extension 130a of the light guide unit 130, which faces the light source element L, curves in accordance with the curve of the emission surface of the light source element L in the vicinity where the extension 130a merges into the rest of the light guide unit 130.
In accordance with the light guide unit 130 described in Japanese Laid-open Publication No. 8-166513 (see FIG. 13A), however, the light emitted from the light source element L creates light rays which are scattered over a wide range of angles within the light guide unit 130 at the curved portion of the transmission adjusting section 131, which is formed of white plastic. Therefore, in accordance with the light guide unit 130, as shown in FIG. 14, some of the light rays which have been scattered by the curved portion of the transmission adjusting section 131 and reached the upper face of the light guide unit 130 may have an acute incident angle with respect to the upper face of the light guide unit 130.
The critical incident angle (xcex8a) is obtained from the following formula according to Shell""s law:
xcex8a=sinxe2x88x921 n/nD.
Assuming that the refractive index n of the air is 1.0, and the refractive index nD of the light guide unit 130 is 1.5, the critical angle (xcex8a) is calculated to be about 42 degrees from the above formula. In this case, those light rays which strike the upper face of the light guide unit 130 with an incident angle larger than about 42 degrees are all reflected at the upper face of the light guide unit 130 back into the light guide unit 130. On the other hand, among those light rays with incident angles smaller than the critical angle (xcex8a), some are reflected at the upper face of the light guide unit 130 back into the light guide unit 130, while others may proceed straight through, or be refracted at, the upper face of the light guide unit 130. The light rays which have thus gone directly out of the light guide unit 130 create bright lines 132. Due to these bright lines 132, a different amount and/or direction of light may be obtained in regions of the upper face of the light guide unit 130 adjacent to the curved portion of the transmission adjusting section 131, as compared to the other regions of the upper face of the light guide unit 130.
As shown in FIG. 15, the luminance at the upper face of the light guide unit 130 is constant over the flat portion of the transmission adjusting section 131 (shown at the left end of FIG. 15), then increases over the vicinity of the curved portion due to the bright lines 132 directly going out from the transmission adjusting section 131, and decreases toward the center of the light guide unit 130 due to the total reflection occurring at the upper face.
When observing the upper face of the light guide unit 130, the viewing angle which produces the maximum luminance also differs between the flat portions and the curved portion of the transmission adjusting section 131. As shown in FIGS. 16A and 16B, at point A, where the transmission adjusting section 131 is flat, the highest luminance to obtained when the upper face of the light guide unit 131 is observed in a direction perpendicular to the upper face of the light guide unit 130. On the other hand, at point B, where the transmission adjusting section 131 is curved, the highest luminance is obtained when the light guide unit 130 is observed from the center of the light guide unit 130 toward the light source element L, i.e., when the viewing angle matches the outgoing angle of the bright lines 132.
Because these phenomena exist, when a backlighting device incorporating the light guide unit 130 and the light source element L is applied to a liquid crystal display apparatus, some luminance diversity may be observed along the light source element L when the edge portion (where the light source element L is provided) is viewed from the center of the liquid crystal display apparatus.
Also, due to the curved portion of the transmission adjusting section 131 covering the top of the light source element L, the light rays which should longitudinally enter the light guide unit 130 are also diffused by the transmission adjusting section 131. This reduces the utilization efficiency of the light from the light source element L, and thus the total luminance at the upper face of the light guide unit 130 is reduced. With reference to FIGS. 17A and 17B, the degree of luminance reduction which may be caused by the curved portion of the transmission adjusting section 131 which covers the top of the light source element L, and the luminance level which may be obtained by solving this problem, will be described.
In FIG. 17A, D1 is an effective light guiding cross-sectional length (along the height direction) of the light guide unit 130 over which the light emitted from the light source element L directly enters the light guide unit 130 along the longitudinal direction without any luminance loss. D2 is a semi-effective light guiding cross-sectional length (along the height direction) of the light guide unit 130 over which the light emitted from the light source element L is diffused by the transmission adjusting section 131 so as to cause some decrease in the luminance. DO represents an effective light guiding cross-sectional length (along the height direction) over which the light emitted from the light source element L can enter the light guide section 130 without any luminance loss in a hypothetical case where there is no luminance reduction of the light over the length indicated as the semi-effective light guide cross-sectional length D2.
The luminance at the surface of the light guide unit 130 created by the light emitted from the light source element L is uniquely determined by the ratio of the effective light guiding cross-sectional length to the diameter of the light source element L. FIG. 17B is a graph representing the relationship between the luminance at the upper face of the light guide unit 130 and the ratio of the effective light guiding cross-sectional length to the diameter of the light source element L. The vertical axis of the graph represents relative luminance values at the surface of the light guide unit 130, where the luminance of the surface of the light guide unit 130 when the effective light guiding cross-sectional length is equal to the diameter of the light source element L is defined as 100%. The horizontal axis of the graph represents the ratio of the effective light guiding is cross-sectional length to the diameter of the light source element L. The luminance at the surface of the light guide unit 130 can be determined from this graph based on a sum total of its semi-effective light guiding cross-sectional length and the effective light guiding cross-sectional length.
In the light guide unit 130, the transmission adjusting section 131 formed of white plastic hinders the entrance of the light from the light source element L into the light guide unit 130. Therefore, it is assumed that the light amount per unit length which is introduced over the semi-effective light guiding cross-sectional length D2 is less than 50% of the light amount per unit length which is introduced over the effective light guiding cross-sectional length D1. Both the semi-effective light guiding cross-sectional length D2 and the effective light guiding cross-sectional length D1 are of the same value, i.e., 1.5 mm. Thus, assuming that the light amount introduced over the semi-effective light guiding cross-sectional length D2 is less than 50% of the light amount introduced over the effective light guiding cross-sectional length D1, the total length for the effective light guiding cross-sectional length in the light guide unit 130 is about 2.3 mm. If the diameter of the light source element L is 2.4 mm, the ratio of the cross-sectional length for effective light to the diameter of the light source element is 0.96, by applying this value to the graph, the luminance at the surface of the light guide unit 130 is determined to be about 95%.
On the other hand, if the transmission adjusting section 131 did not interfere with the light entering the light guide unit 130, the light emitted from the light source element L would directly enter the light guide unit 130 along the longitudinal direction, without any luminance loss, over the effective light guiding cross-sectional length (along the height direction) D0, which is 3.0 mm. In this case, the ratio of the effective light guiding cross-sectional length to the diameter of the light source element is 1.25. By applying this value to the graph, the luminance at the surface of the light guide unit 130 is determined to be about 120%.
Accordingly, by preventing the luminance reduction caused by the transmission adjusting section 131, the luminance at the surface of the light guide unit 130 can be improved by more than about 20%.
As described above, the light guide unit 130 has the problems of luminance diversity observed around the light source element L, and the decrease of the luminance in the light guide unit 130.
Furthermore, a light guide unit of a liquid crystal display apparatus is generally formed of acrylic resin having a heat distortion temperature of about 95xc2x0 C. The peripheral area of the electrodes of light source element may rise up to about 100xc2x0 C., at which temperature the resin material composing the light guide unit undergoes plastic deformation, causing light leakage at the peripheral portion of the frame of the backlighting device or misalignment of the optical sheet.
A backlighting device according to the present invention includes: a light guide unit formed as a substantially flat panel of a first resin material having opposing surfaces and side faces, the light guide unit having a light guide section, wherein light enters the light guide unit through at least one of the side faces and is emitted from one of the opposing surfaces; and a light source element disposed adjacent to the at least one side face of the light guide section for irradiating light onto the at least one side face of the light guide section, wherein the light guide unit further includes a light scattering section is formed of a second resin material, the light scattering section substantially perpendicularly protruding from the at least one side face of the light guide unit, and the light scattering section overlies the light source element along a direction of travel of the light exiting from the light guide section so as to scatter the light irradiated by the light source element.
In one embodiment of the invention, the first resin material is a transparent resin material.
In another embodiment of the invention, the second resin material contains a light scattering agent.
In still another embodiment of the invention, the light scattering section is arranged so as to be engaged with the at least one side face of the light guide unit.
In still another embodiment of the invention, the light scattering section is supported by a supporting section protruding integrally from the light guide section, the supporting section being formed of the first resin material.
In still another embodiment of the invention, the second resin material composing the light scattering section has heat resistance which is greater than heat resistance of the first resin composing the light guide section.
In still another embodiment of the invention, the second resin material composing the light scattering section contains about 2% to about 5% by weight of a light scattering agent.
In still another embodiment of the invention, a thickness t of the light scattering section and a total light transmittance T satisfy the formula:
10.1xc3x97EXP(xe2x88x921.406t)xe2x89xa6Txe2x89xa656.3xc3x97EXP(xe2x88x921.569t)
In still another embodiment of the invention, the light guide unit and the light scattering section are integrally formed by injection molding, the first resin material being different from the second resin material.
In still another embodiment of the invention, the second resin material composing the light scattering section after molding has a second contraction ratio, and the first resin material composing the light guide section after molding has a first contraction ratio, the first contraction ratio being different from the second contraction ratio.
In still another embodiment of the invention, the light scattering section and the light guide section are disposed so that the light scattering section clamps the light guide section due to a compressive stress created by a difference between a second contraction ratio of the second resin material composing the light scattering section after molding and a first contraction ratio of the first resin material composing the light guide unit after molding.
In still another embodiment of the invention, the aforementioned backlighting device further includes a rib provided on the at least one side face of the light guide section irradiated by the light from the light source element, the rib supporting a substantially perpendicular corner formed between the at least one side face and the light scattering section.
In still another embodiment of the invention, the rib is formed of the first resin.
In still another embodiment of the invention, the first resin material is a transparent resin material.
In still another embodiment of the invention, the backlighting device includes a plurality of ribs being spaced apart from one another by a distance in a range from about 3 mm to about 50 mm.
In still another embodiment of the invention, the backlighting device further includes at least one light diffusion plate on one of the opposing surfaces of the light guide unit through which the light from the light source element exits.
In still another embodiment of the invention, the backlighting device includes a second diffusion plate overlying the first diffusion plate, the second diffusion plate having a lower haze level than the first diffusion plate.
In another aspect of the invention, there is provided a liquid crystal display device incorporating any one of the aforementioned backlighting devices, wherein light emitted from the backlighting device is irradiated on a liquid crystal panel of the liquid crystal display device.
In yet another aspect of the invention, there is provided a method for manufacturing any one of the aforementioned backlighting devices, including the steps of: forming the light guide section by applying injection molding to the first resin; forming the light scattering section so as to be integral with the light guide section by applying injection molding to the second resin material contains a light scattering agent; and disposing the light source element so as to be adjacent to the light scattering section and the light guide section.
In one embodiment of the invention, the step for producing the light guide section includes forming a first engaging section, and the step for producing the light scattering section includes forming a second engaging section, the first engaging section of the light guide section being engaged with the second engaging section of the light scattering section.
In another embodiment of the invention, the step of forming the light guide section includes forming a rib on the at least one side face for supporting a substantially perpendicular corner formed between the at least one side face and the light scattering section.
Thus, the invention described herein makes possible the advantages of (1) providing a backlighting device which is suitable for mass production, with minimum luminance or color diversity; (2) a method for producing such a backlighting device; and (3) providing a liquid crystal display apparatus incorporating the backlighting device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.